Vertex jumping when moving objects with the mirror modifier
It happens to me time to time as well, but only when I forget to uncheck 'x mirror'in the Tool Panel within the Options Tab. 

If that is checked and you only have half a mesh in the case with a mirror modifier, Blender will try to calculate the missing halves verts by pulling them over to the missing side.

Either way, just uncheck it and things will be back to normal. :)